Biochar and Gypsum Influences on Arsenic Mobility in Paddy Soils.

Arsenic (As) originating from the Himalayas occurs as a natural contaminant in paddy soils and ground water used for irrigation in the large river deltas in South and South-East Asia. In addition to drinking As contaminated water, millions of people in these regions are thereby further exposed to As through elevated levels of As in their staple food rice. Biochar derived from rice residues, especially husks, is a cheap and abundantly available resource in rice farming areas due to its common use as fuel in gasifiers powering the rice mills. As such, it has been proposed as a potential amendment for increasing fertility and crop yield, while decreasing the mobility of toxins and increasing carbon sequestration. So far, results are inconclusive about the benefits of biochar application to paddy rice fields, especially regarding the mobility of As.

In a pot trial, with paddy rice and As contaminated soils from Cambodia, biochar derived from rice husks (RHC) and rice straw (RSC) was tested with or without addition of gypsum, as a means to reduce arsenic (As) uptake in rice. Both chars decreased the As concentration in rice grain and straw compared with unamended pots, whereas gypsum had a very limited effect on the arsenic uptake. The effect was strongest with RSC. A multiple methods approach was employed to decipher the mechanistic explanation for the char effect and led to the conclusion that several processes combine in a heterogeneous spatial and temporal pattern to generate the response.